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The family Leguminosae provides the classical examples of root nodule plants, and owing to the agricultural importance of that family of plants their root nodules have received continuous investigation for more than 50 years. Though no other large family is similarly characterised by the possession of root nodules, such structures are shown by a number of genera which occur in curiously scattered fashion through the Phanerogams, the only other particular feature in common being that the plants concerned are practically all of woody type. These non-legumes have been the subject of sporadic investigation, chiefly from the standpoint of the identity of the endophytes. Three nodulated genera represented in Europe are those mentioned in the title. These plants are abundant in the appropriate habitats, and the question of the functional significance of their nodules is of ecological and general interest.The species to which this paper refers are Alnus glulinosa (L.) Gaertn., Myrica gale L. and Hippopha~ rhamnoides L. In addition to the function of the nodules, the extent to which their development and activity are affected by environmental factors will also be considered. The comparative treatment of the three species will be based partly on new observations and partly on data already published by the authors in respect of individual species. Aspects which will not be specifically considered include the structure and cytology of the nodules, and the systematic position of the endophytes. In the last connection it may be noted that the endophytes *) Now of the West

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